1. Field of the Invention
The present invention relates generally to microwave motion sensing apparatus and more particularly to an improved sensing system which uses pulsed microwave transmission and digital type filtering to substantially improve its efficiency and operational accuracy.
2. Description of the Prior Art
Prior art microwave motion sensing devices consist of a simple transmit and receive module or modules followed by a bandpass amplifier and signal processing circuitry. The system includes a DC power input that is used to energize a mirowave source and set up a mirowave oscillation which is transmitted into a detection area to be monitored. A portion of the transmitted energy will be reflected from the surrounding environment and any moving objects within the monitored field and will be returned to the source where it is received in the receiving portion of the device. The received energy is detected as a steady state DC level representing the reflection of any static objects plus a small AC, Doppler component representing any motion within the detection area. The detected output is then AC coupled into a bandpass amplifier having a bandpass of about 5 to 90 Hz so as to remove the DC components therefrom. The amplifier provides the gain required to bring the signal up to whatever level is necessary to indicate that there is or is not motion within the scanned region.
One of the disadvantages of such circuitry is that it requires the application of continuous power to the unit, typically on the order of 12 volts at about 150 milliamps, or about 1.8 watts of power. It therefore requires the provision of a fairly large and expensive auxiliary battery to assure operation of the system for a suitable duration in the event of line power failure.
Another problem with the prior art circuits relates to interference caused by the power supply frequency. Although the 60 Hz frequency itself is not much of a problem and can easily be dealt with, other components higher in frequency provide very annoying sources of interference. One of the most troublesome sources is fluorescent lighting fixtures disposed within the detection field. Since such fixtures have a discharge frequency of 120 Hz (twice the power supply frequency) every time the gas in a fluorescent tube ionizes, it causes the equivalent of a metallic conductor to appear in and then disappear from the detection field at a rate (120 Hz) which corresponds to motion at approximately 4 mph or within the walking range of a human. Although an accurately adjusted bandpass characteristic of the amplifier can filter out this frequency, due to the limitations of conventional analog circuitry, obtaining and maintaining desirable accuracy is difficult and requires manual adjustment. Consequently, the detection units must be very carefully placed so as not to be influenced by fluorescent lights, or alternatively, the system must only be used when the fluorescent lighting fixtures are turned off. Other similar sources of interference include motors, fans, etc., which are typically synchronized to 60 Hz or multiples thereof.